21.1 Magnets and Magnetic Fields
All magnets have two magnetic poles, regions where the magnet’s force is strongest.
• One end of a magnet is its north pole.• The other end is its south pole. • The direction of the magnetic force between two
magnets depends on how the poles face.
Magnetic Forces
21.1 Magnets and Magnetic Fields
A magnetic field surrounds every magnet. Iron filings reveal the field lines, which start near the north pole and extend toward the south pole.
Magnetic Fields
• Where lines are close together, the field is strong. • Where lines are more spread out, the field is weak.
21.1 Magnets and Magnetic Fields
A. When like poles of two magnets come together, the magnets repel each other.
B. When opposite poles of magnets come together, the magnets attract each other.
Magnetic Fields
21.1 Magnets and Magnetic Fields
If you cut a magnet in half, each half will have its own north pole and south pole because the domains will still be aligned.
A magnet can never have just a north pole or just a south pole.
Magnetic Materials
21.1 Magnets and Magnetic Fields
A magnetic field can magnetize ferromagnetic materials. A. Before magnetization, domains are random.
B. Domains aligned with the field grow during magnetization. Unaligned domains can shrink.
Magnetic Materials
21.1 Magnets and Magnetic Fields
Electricity and magnetism are different aspects of a single force known as the electromagnetic force.
• The electric force results from charged particles.
• The magnetic force usually results from the movement of electrons in an atom.
Electricity and Magnetism
21.1 Magnets and Magnetic Fields
Magnetic Fields Around Moving Charges
Moving charges create a magnetic field.• Moving charges may be the vibrating charges that
produce an electromagnetic wave or the moving charges in a wire.
• Magnetic field lines form circles around a straight wire carrying a current.
Electricity and Magnetism
21.1 Magnets and Magnetic Fields
If you point the thumb of your right hand in the direction of the current, your fingers curve in the direction of the magnetic field.
Electricity and Magnetism
Direction of electron flow
Direction of current
Current-carrying wire
Direction of magnetic field
21.1 Magnets and Magnetic Fields
A coil of current-carrying wire that produces a magnetic field is called a solenoid.
Solenoids and Electromagnets
The magnetic field lines around a solenoid are like those of a bar magnet.
21.1 Magnets and Magnetic Fields
he strength of an electromagnet can be increased using the following methods.
• Increase the current flowing through the solenoid.
• Increase the number of turns.• Adding a metal core.
Solenoids and Electromagnets
21.1 Magnets and Magnetic Fields
Electric Motors
An electric motor is a device that uses an electromagnet to turn an axle.
Electromagnetic Devices
21.1 Magnets and Magnetic Fields
Assessment Questions
1. Where does the magnetic field of a magnet have the strongest effect on another magnet? a. the north pole
b. the south pole
c. both poles equally
d. midway between the two poles
21.1 Magnets and Magnetic Fields
Assessment Questions
1. Where does the magnetic field of a magnet have the strongest effect on another magnet? a. the north pole
b. the south pole
c. both poles equally
d. midway between the two poles
ANS: C
21.1 Magnets and Magnetic Fields
Assessment Questions
2. What happens to a permanent magnet if its magnetic domains lose their alignment? a. The magnetic field reverses direction.
b. It loses its magnetic field.
c. It has several north poles and several south poles.
d. It is no longer a ferromagnetic material.
21.1 Magnets and Magnetic Fields
Assessment Questions
2. What happens to a permanent magnet if its magnetic domains lose their alignment? a. The magnetic field reverses direction.
b. It loses its magnetic field.
c. It has several north poles and several south poles.
d. It is no longer a ferromagnetic material.
ANS: B
21.1 Magnets and Magnetic Fields
Assessment Questions
3. Which change will increase the strength of an electromagnet made by wrapping a conductive wire around an iron nail? a. reversing the direction of current flowb. replacing the nail with a wooden dowelc. increasing the number of coils of wire around the naild. using a longer nail
21.1 Magnets and Magnetic Fields
Assessment Questions
3.Which change will increase the strength of an electromagnet made by wrapping a conductive wire around an iron nail? a. reversing the direction of current flowb. replacing the nail with a wooden dowelc. increasing the number of coils of wire around the naild. using a longer nail
ANS: C